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Peripheral Nervous System (PNS)
Comprehensive notes on PNS Classification, Sympathetic & Parasympathetic Systems, Cranial, and Spinal Nerves for Pharmacy Exams.
The Nervous System is divided into the Central Nervous System (CNS - Brain & Spinal Cord) and the Peripheral Nervous System (PNS). The PNS consists of all nervous tissue outside the CNS (Nerves, Ganglia, Enteric plexuses, and Sensory receptors).
3 Main Divisions of PNS:
- Somatic Nervous System (SNS): Controls Voluntary actions. It conveys output from the CNS to skeletal muscles only.
- Autonomic Nervous System (ANS): Controls Involuntary actions. Conveys output from the CNS to smooth muscles, cardiac muscle, and glands.
- Enteric Nervous System (ENS): Known as the "Brain of the gut". It involuntarily governs the function of the gastrointestinal (GI) tract.
The Autonomic Nervous System (ANS) has two main branches. Most organs have Dual Innervation, meaning they receive impulses from both branches, which usually have opposing (antagonistic) effects.
| Feature | Sympathetic Nervous System | Parasympathetic Nervous System |
|---|---|---|
| Function | "Fight-or-Flight" response. Prepares body for emergency, stress, or exercise. | "Rest-and-Digest" response. Conserves energy and promotes housekeeping functions. |
| Anatomical Origin | Thoracolumbar Outflow (T1 to L2 segments of spinal cord). | Craniosacral Outflow (Cranial nerves III, VII, IX, X and Sacral S2-S4). |
| Heart Rate | Increases (Tachycardia). | Decreases (Bradycardia). |
| Airways (Lungs) | Dilates (Bronchodilation to get more air). | Constricts (Bronchoconstriction). |
| Neurotransmitter | Releases Noradrenaline at the effector organ. | Releases Acetylcholine (ACh) at the effector organ. |
There are 31 pairs of spinal nerves that emerge from the spinal cord through the intervertebral foramina. They connect the CNS to sensory receptors, muscles, and glands in all parts of the body.
Regions (31 Pairs)
- 8 pairs of Cervical nerves (C1–C8)
- 12 pairs of Thoracic nerves (T1–T12)
- 5 pairs of Lumbar nerves (L1–L5)
- 5 pairs of Sacral nerves (S1–S5)
- 1 pair of Coccygeal nerves (Co1)
Structure (Roots)
Each spinal nerve connects to the spinal cord via two roots:
- Posterior (Dorsal) Root: Contains Sensory nerve fibers carrying impulses from the body to the CNS.
- Anterior (Ventral) Root: Contains Motor nerve fibers carrying impulses from the CNS to the muscles/glands.
There are 12 pairs of cranial nerves that emerge directly from the brain/brainstem. They pass through various foramina in the skull.
Exam Trick to Remember 12 Cranial Nerves:
"Oh Oh Oh To Touch And Feel Very Good Velvet, Ah Heaven!"
O: Olfactory | O: Optic | O: Oculomotor | T: Trochlear | T: Trigeminal | A: Abducens | F: Facial | V: Vestibulocochlear | G: Glossopharyngeal | V: Vagus | A: Accessory | H: Hypoglossal
| No. | Name | Type (Sensory/Motor/Both) | Key Function |
|---|---|---|---|
| I | Olfactory | Sensory | Smell |
| II | Optic | Sensory | Vision (Sight) |
| III | Oculomotor | Motor | Movement of eyelid & eyeball |
| IV | Trochlear | Motor | Movement of eyeball |
| V | Trigeminal | Both | Chewing, sensation of face |
| VI | Abducens | Motor | Movement of eyeball |
| VII | Facial | Both | Facial expression, taste, saliva |
| VIII | Vestibulocochlear | Sensory | Hearing and Equilibrium (Balance) |
| IX | Glossopharyngeal | Both | Swallowing, taste, saliva secretion |
| X | Vagus | Both | Smooth muscle control of GI tract & Heart |
| XI | Accessory | Motor | Movement of head & shoulders |
| XII | Hypoglossal | Motor | Movement of tongue during speech |
Clinical Correlation (Nerve Disorders)
- Bell's Palsy: Damage to the Facial nerve (Cranial Nerve VII), causing paralysis of facial muscles on one side of the face.
- Sciatica: Severe pain along the sciatic nerve (the longest nerve in the body), usually caused by a herniated disc compressing the lumbar/sacral spinal nerve roots.
- Neuropathy: Any disease of the peripheral nerves, often seen in long-term diabetes, causing numbness and tingling in hands and feet.